The present invention relates to hydrodynamic fluid couplings, and more particularly to improvements in controllable-filling fluid couplings whose characteristic curves (i.e., the torque transmitted by the coupling in dependency on the ratio of runner speed n.sub.T to impeller speed n.sub.p at different degrees of filling of the working circuit with working liquid) continuously slope downwardly as the ratio n.sub.T /n.sub.p increases. Fluid couplings which exhibit such characteristics are disclosed, for example, on page 18 of the German publication entitled VDI-Richtlinien VDI 2153, Apr. 1974. Means for achieving such characteristics may include a flow restrictor which is installed in the radially inner region of the working circuit. Reference may be had to German Pat. No. 587,616.
As a rule, the extent to which the working circuit of a hydrodynamic fluid coupling is filled with working liquid is regulated by changing the rate of liquid flow from the working circuit. Such mode is preferred to the method according to which the degree of filling is regulated by changing the rate of liquid admission into the working circuit because the regulation of evacuation of liquid insures a more satisfactory withdrawal of heat during each stage of operation of the fluid coupling. As a rule, the means for evacuating working liquid comprises an adjustable scoop tube whose scooping orifice extends into a so-called scoop chamber which is a rotatable liquid reservoir chamber in communication with the working circuit. However, controlled evacuation can also be achieved by other means, e.g., by arranging radially extending openings, which are adjustable by a valve member, on the impeller of the fluid coupling. Reference may be had to German Pat. No. 1,600,974.
German patent No. 1,121,888 discloses a fluid coupling which is provided with an adjustable scoop tube (see FIGS. 5-7 of the patent) and includes means for effecting a controlled acceleration of the load. The controller means includes an actuator cylinder whose piston is connected with the scoop tube. One end face of the piston is adjacent to a cylinder chamber which can receive pressurized fluid serving to move the scoop tube toward the "circuit empty" position. The other end face of the piston is biased by a spring which tends to move the scoop tube toward the "circuit full" position. The cylinder chamber is connected to an inlet pipe which supplies liquid to the working circuit. The patent further proposes to replace the spring with a weighted lever. When the load is to be accelerated, the scoop tube initially assumes the "circuit full" position (while the inlet tube and the working circuit are still empty). As the inlet pipe begins to admit liquid into the working circuit, a portion of such liquid flows into the chamber of the actuating cylinder for the scoop tube and causes an abrupt movement of the scoop tube to the "circuit empty" position. The aforementioned spring or weighted lever thereupon begins to move the scoop tube back toward the "circuit full" position. The just described mode of acceleration of the load results in delayed filling of working circuit with liquid, i.e., it provides a longer interval of acceleration of the load. The speed of movement of the scoop tube toward the "circuit full" position varies (increases); this is intended to insure that the transmitted torque remains at least substantially constant. It has been found that the just described mode of effecting movements of the scoop tube does not prevent excessive deviations of transmitted torque from an optimum value. Moreover, the instant surge of torque at the start of acceleration (when the scoop tube still assumes the "circuit full" position) is often undesirable and/or damaging.
It was also proposed (see German OS No. 2,438,132) to provide the fluid coupling with a controller system which effects a continuous increase of the quantity of liquid in the working circuit as long as the transmitted torque does not reach a predetermined value and which prevents an increase of the quantity of liquid in the working circuit as long as the torque exceeds or equals the predetermined value. The torque is monitored by measuring the current consumption of the motor which drives the impeller of the fluid coupling. A drawback of such proposal is that the controller system is complex, expensive and prone to malfunction. This is attributable to the fact that the progress of characteristic curves which represent variations of torque at different degrees of filling of the working circuit varies considerably, i.e., certain curves are relatively flat all the way, certain curves include flat portions flanking downwardly sloping intermediate portions, and certain curves exhibit a steep downward inclination from one end to the other.